System and method for bone preparation for an implant

ABSTRACT

A computer assisted surgical system for guiding bone cutting operations of an arthroplasty surgery is provided. The computer assisted surgical system includes a cutting guide having a guiding surface for mounting to a bone and a computer. The computer is in communication with the cutting guide and configured to generate a bone model of the first bone without the use of pre-operative bone images, determine a position of an implant model having a bone interface surface on the bone model, determine a position of the cutting guide on the first bone based on the determined position of the implant model on the bone model, and position the guiding surface of the cutting guide to one of a plurality of predetermined cut configurations relative to the bone interface surface.

BACKGROUND OF THE INVENTION

The present invention relates to an improved system and method for bonepreparation for an implant. In particular, the present invention relatesto a computer assisted surgical system and method for resecting bone toreceive an orthopedic implant for press-fit applications.

Arthroplasty or joint replacement surgical procedures require accuratebone preparation for the implantation of prosthetic components i.e.,implants. A precise fit between the implant and resected bone is desiredfor good fixation in both cementless and cemented applications in orderto prevent loosening of the implant over time. Furthermore, aninterference fit or “press-fit” is often desired to ensure good fixationof the implant to the bone, and for osteointegration of cementlessdevices. The amount of interference fit to apply, however, is difficultto ascertain and often determined by empirical methods with the benefitof intra-operative evaluations of the patient's bone.

For example, in a total knee joint replacement surgery, the distalfemur, proximal tibia and patellar articulating surfaces are replacedwith prosthetic components. The femoral component typically requiresthat the distal femur be resected with e.g., five bone cuts that aremade at angles to one another when viewed from a sagittal perspective.The bone cuts are commonly performed with a bone cutting tool, such as asaw or a mill that is guided by a manual cutting block or a template.

Surgical cutting blocks can have integrated cutting slots or opensurfaces that correspond to the inner shape of the implant to be used.These guiding surfaces are at fixed distances and angles from oneanother so that one cutting block must be provided for each size andshape of the implant or a limited range of sizes of the type of implant.

Implants used in cementless applications often incorporate a porouscoating about its bone interface surface. The porous coating istypically structured so as to be dimensionally offset from the implant'sbone interface surface. That is, the porous coating typically sits proudof the implant's bone interface surface. However, regardless of whetheror not an implant is to be used for cemented or cementless applications,the bone preparation i.e., bone cuts are typically the same. That is,the same cutting blocks/jigs are used to guide the bone cuts forimplants used for either cementless or cemented applications. Therefore,the resulting discrepancy between the bone cut formed using the cuttingblock and the internal bone interface surface of the implant (which maybe at least in part determined by the thickness of the coating) providesa nominal degree of press-fit interference between the bone cuts asestablished by the cutting block and the implant once it is impactedonto the resected bone. Alternatively, a manual cutting block can beused with cutting planes that are slightly offset and/or angled from theinner surfaces of the implant to achieve a predetermined and fixedamount of interference.

One of the challenges orthopaedic surgeons face in the operating room isthe varying quality of bone they are presented with at surgery. Patientsundergoing joint replacement surgery typically suffer from degenerativeconditions that can alter bone quality. Thus, from one patient toanother, the surgeon may encounter varying degrees of bone quality. Forexample, upon exposing the patient's joint and bones, the surgeon mayencounter bone that is very poor in quality, having a relatively lowdensity or high porosity. On other occasions, for example, in youngermore active patients, the bone can be relatively dense and hard. Thequality of bone can affect the degree or tightness of the fit of theimplant for a given amount of interference. Hence, when bone cuts areprepared in the same fashion for all patients, the resulting tightnessor degree of the fit can vary from one patient to another depending uponthe bone quality, which may ultimately impact the overall short term andlong term success of the implant. However, there are currently no toolsavailable that allows the surgeon to precisely and objectively adjustthe bone cuts during a surgical procedure in order to customize thedegree of press-fit interference between the implant and resected bonefor individual patients that takes into account the varying quality ofbone of individual patients.

Another disadvantage of conventional systems used in arthroplasty isthat they lack flexibility in the ability to adjust the relativepositions of the bone cuts for any particular implant which can allowfor varying the resulting degree of interference press-fit achievedbetween the resected bone and the implant being used. Additionally,conventional systems cannot provide multiple levels of interferencepress-fit in an all-in-one device. For example, in a manual set ofinstruments, once the cutting blocks are manufactured they cannot bechanged and the surgeon needs to accept those relative cut locations forall patients and surgical circumstances.

Therefore, a need still exists for a system and method for thepreparation of bone to receive an implant that can provideintra-operative flexibility to a surgeon user and allow for variationsin the degree of press-fit interference provided based upon pre orintra-operative assessment of the patient e.g., bone quality and implanttrialing. Such a need is met by the system and method of the presentinvention.

BRIEF SUMMARY OF THE INVENTION

In an embodiment, the present invention provides a computer assistedsurgical system for guiding bone cutting operations of an arthroplastysurgery including a cutting guide and a computer. The cutting guide hasa guiding surface and mounts to a bone. The computer is in communicationwith the cutting guide, which can be a robotic cutting guide. Thecomputer is configured to intra-operatively generate a bone model of thebone without the use of pre-operative bone images, determine a positionof an implant model having a bone interface surface on the bone model,determine a position of the cutting guide on the bone based on thedetermined position of the implant model on the bone model, and positionthe guiding surface of the cutting guide to one of a plurality ofpredetermined cut configurations relative to the bone interface plane.The bone model can also be generated intra-operatively.

In another embodiment, the present invention provides a method ofpreparing a first bone for an implant during an arthroplasty. The methodincludes generating a bone model of the first bone and providing animplant model having a bone interface. An optimum position of theimplant model on the bone model is then determined and a quality of thefirst bone is intra-operatively evaluated. The method further includesselecting a degree of interference fit between the implant model and thebone model based on the intra-operative evaluation of the quality of thefirst bone and mounting a cutting guide having a cutting guide surfaceon the first bone based on the determined optimum position. The cuttingguide surface is positioned to one of a plurality of predetermined cutconfigurations that corresponds to the selected degree of interferencefit.

In yet another embodiment, the present invention provides a method ofpreparing a first bone for an implant during an arthroplasty. The methodincludes providing a computer assisted surgery system configured togenerate a bone model of the first bone. The computer assisted surgerysystem includes a memory having a plurality of implant models and a setof default user preferences for resecting bone based on a selectedimplant model. The method further includes selecting an implant modeland determining an optimum position of the implant model on the bonemodel and attaching a cutting guide in communication with the computerassisted surgery system to the first bone. Further, the method includesautomatically aligning the cutting guide to correspond to the defaultuser preferences for resecting bone based on the selected implant modeland intra-operatively evaluating a quality of the first bone. Then it isdetermined if a repositioning of the cutting guide from the default userpreferences is required.

In a further embodiment, the present invention provides a computerassisted surgical system for guiding bone cutting operations of anarthroplasty surgery that includes a cutting guide and a computer. Thecutting guide is to be positioned relative to a first bone. The computerhas a plurality of predetermined cut configurations that corresponds tovarious degrees of interference fit of a prosthesis on the first boneand is in communication with the cutting guide. The computer is alsoconfigured to position the cutting guide to one of the plurality ofpredetermined cut configurations.

In yet a further embodiment, the present invention provides a method ofpreparing a bone for an implant during an arthroplasty. The methodincludes providing a computer assisted surgery system having a pluralityof predetermined cut configurations and selecting a degree ofinterference fit between an implant and a bone that corresponds to oneof the plurality of predetermined cut configurations. A cutting guide isthen positioned to one of a plurality of predetermined cutconfigurations corresponding to the selected degree of interference.

In another embodiment, the present invention provides a method ofpreparing a first bone for an implant during an arthroplasty. The methodincludes providing a computer assisted surgery system having a set ofdefault user preferences for resecting bone that corresponds to aninterference fit of a prosthesis on the first bone and positioning acutting guide in communication with the computer assisted surgery systemrelative to the first bone. The method further includes aligning thecutting guide to correspond to the default user preferences forresecting bone and intra-operatively evaluating a quality of the firstbone. Then it is determined if a repositioning of the cutting guide fromthe default user preferences is required.

In yet another embodiment, the present invention provides a roboticcutting guide for guiding bone cutting operations of an arthroplastysurgery that includes a guiding member and a plurality of predeterminedcut configurations that corresponds to various degrees of interferencefit of a prosthesis on the bone. The guiding member is positionedrelative to a bone. The robotic cutting guide is also configured tointra-operatively receive a first selection of one of the plurality ofpredetermined cut configurations and move the guiding member to thefirst selected predetermined cut configuration. The robotic cuttingguide can also receive a second selection of one of the plurality ofpredetermined cut configurations and move the guiding member to thesecond selected predetermined cut configuration.

In accordance with a first aspect, the present invention also provides acomputer assisted surgical system for cutting bone that includes a guideand a navigation system in communication with the guide. The guideguides a cutting tool for cutting bone. The guide is pivotably mountableto bone about a first axis of the guide, and includes a saw guidecoupled to a second axis of the guide. The navigation system includes adisplay and a system controller in communication with the display. Thesystem controller is configured to store in memory dimensional data andstandard cutting plane data associated with an implant to be implantedto the bone, receive input for a desired bone resection associated witha desired degree of implant-bone interference for a press-fitapplication, position the saw guide in alignment with or at an off-setwith the standard cutting planes based upon the received input for thedesired bone resection, and display image and dimensional data of theimplant in combination with the bone based upon the inputted desiredbone resection.

In a second aspect, the present invention provides a method of preparingbone for receiving an implant. The method includes the steps ofattaching a pivotable guide to a bone, assessing a desired degree ofpress-fit interference between the implant and the bone, inputting thedesired degree of bone resection based upon the assessed desired degreeof press-fit interference to a navigation system associated with thepivotable guide and positioning the pivotable guide to a position basedupon the inputted desired degree of bone resection. The method canfurther include the steps of resecting bone with the pivotable guide,assessing an interference fit between the implant and resected bone,inputting another desired degree of bone resection based upon theassessed press-fit interference between the implant and the bone, andrepositioning the pivotable guide to a subsequent position based uponthe inputted desired degree of bone resection after the assessment ofthe press-fit interference. Furthermore, the method can include the stepof assessing a desired degree of press-fit interference between theimplant and the bone based upon bone quality data of the bone.

In a third aspect, the present invention provides a flexible system thatgives a surgeon the ability to intra-operatively customize bone cutsaccording to the surgeon's preference and/or individual patient factors.

A fourth aspect, the present invention provides the ability tointra-operative select the degree of press-fit interference either atthe beginning of or during the surgical procedure.

A fifth aspect of the present invention provides the ability to selectfrom a plurality of cut configurations that correspond to varyingamounts of congruency or interference between the inner implant surfaceand outer surface of the resected or cut bone.

A sixth aspect of the present invention provides the ability to modifythe degree of press-fit interference or congruency on planar bone cuts(e.g., bone cuts made during a TKA) as well as for curved bone cuts(e.g., during a resurfacing procedure). For example: 0 degrees or 0 mmof press-fit, ¼, ¾, 1, 1¼, etc. degrees or mm can be applied and theability to vary a continuous parameter that changes the amount ofpress-fit interference (e.g., X degrees or mm).

A seventh aspect of the present invention provides the ability to assessthe quality of the patient's bone and to use this information in adecision making process in order to select the appropriate amount ofpress-fit interference that is most optimal for a specific patient.

An eighth aspect of the present invention provides the ability toperform one or more resections before deciding which bone cutconfiguration to use (for example a distal cut, or an anterior cut).

A ninth aspect of the present invention provides the ability to assessthe amount of bone coverage between an implant and bone surface in orderto determine how much press-fit interference to apply to the patientbased on the patient's native morphology and the planned positioning ofthe femoral implant component.

A tenth aspect of the present invention provides a display of a colormap on an implant planning screen (superimposed over the modeled bonecut-surfaces) that is indicative of the intensity of the currentlyselected interference fit between the resected bone and the implant.

An eleventh aspect of the present invention provides a system fordetermining an optimal degree of fit between an implant and a resectedbone without requiring any pre-operative medical images of the bone,such as CT, MRI or X-rays.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings an embodiment which is presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown.

In the drawings:

FIG. 1 is a perspective view of a computer assisted surgical system inaccordance with a preferred embodiment of the present invention;

FIG. 2 is a perspective view of a robotic guide applicable to a totalknee arthroplasty in accordance with a preferred embodiment of thecomputer assisted surgical system of FIG. 1;

FIG. 3 is a sagittal view of a conventional femoral implant;

FIG. 4 is a sagittal view of a resected distal femur bone for receivingthe femoral implant of FIG. 3;

FIG. 5 is a sagittal view of the distal femur bone of FIG. 4 assembledto the femoral implant of FIG. 3;

FIG. 6 is a sagittal view of an exemplary set of bone cuts to a distalfemur made with the robotic guide of FIG. 2 in accordance with a methodof the present invention;

FIG. 7 is a screen shot view of an implant planning screen on a displayof the computer assisted surgical system of FIG. 1; and

FIG. 8 is a flow chart of a method of preparing a bone in accordancewith another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the present embodiments of theinvention illustrated in the accompanying drawings. Wherever possible,the same or like reference numbers will be used throughout the drawingsto refer to the same or like features. It should be noted that thedrawings are in simplified form and are not drawn to precise scale. Inreference to the disclosure herein, for purposes of convenience andclarity only, directional terms such as top, bottom, above, below anddiagonal, are used with respect to the accompanying drawings. Suchdirectional terms used in conjunction with the following description ofthe drawings should not be construed to limit the scope of the inventionin any manner not explicitly set forth. Unless specifically set forthherein, the terms “a,” “an” and “the” are not limited to one element butinstead should be read as meaning “at least one.” The term “distal”means situated away from the point of origin or attachment and the term“proximal” means situated toward the point of origin or attachment. Theterminology includes the words noted above, derivatives thereof andwords of similar import.

In a first preferred embodiment, the present invention provides acomputer assisted surgical system (CAS) 10 (FIG. 1) for guiding bonecutting operations of an arthroplasty surgery. The CAS 10 includes arobotic guide positioning device (also referred to as a robotic cuttingguide, a robotic guide, or cutting guide) 50 for guiding a cutting toolfor cutting bone. The cutting guide 50 is preferably in communicationwith and operably connected to the CAS 10, e.g., in communication with acomputer 100 of the CAS 10. That is, the cutting guide 50 can be wiredto the computer 100 or remotely controllable by the computer 100 to moveand guide the cutting guide 50. Exemplary examples of total kneereplacement manual and robotic bone-cutting guides applicable to thepresent invention are disclosed in U.S. Pat. Nos. 5,817,097; 4,574,794;5,571,110 and 6,554,837, and U.S. Patent Application Publication Nos.2010/0130986, 2011/0130761, and 2006/0200161, the disclosures of whichare hereby incorporated by reference herein in their entirety. Exemplaryexamples of computer assisted surgical systems or navigation systems aredisclosed in U.S. Patent Application Publication Nos. 2007/0185498,2007/0106128, 2006/0161052, 2009/018445, 2011/0069867 and 2004/0127788and U.S. Pat. Nos. 6,738,656; 6,527,443; 6,226,548 and 5,682,886, andInternational Application Publication Nos. WO2006/106419 and WO99/60939,the entire disclosures of which are hereby incorporated by referenceherein in their entirety.

The present invention is applicable to any arthroplasty surgery.However, for exemplary purposes only, the present invention willhereinafter be described with reference to a total knee arthroplastysurgery.

FIG. 3 illustrates a side (sagittal) view of a conventional femoralprosthesis/implant 2 applicable to a total knee arthroplasty. Thefemoral prosthesis 2 includes internal or inner surfaces 4 that areintended to directly engage or be fixed to a distal aspect of a resecteddistal femur bone 1, as shown in FIGS. 4 and 5. The inner surfaces 4 aremade up of a plurality of planar cuts, in this case five planarsurfaces, including a distal cut surface 10 a, an anterior chamfer cutsurface 11 a, an anterior cut surface 13 a, a posterior chamfer cutsurface 12 a, and a posterior cut surface 14 a. Referring to FIG. 6, inorder to fit the femoral prosthesis 2 onto the resected distal femurbone 1, the femoral bone is prepared with a series of planar cuts thatcorrespond to the inner surfaces 4 of the femoral prosthesis 2, namely adistal cut 10 b, anterior chamfer cut 11 b, anterior cut 13 b, posteriorchamfer cut 12 b, and posterior cut 14 b (FIG. 6). In this case, thedistal femoral bone cuts 10 b, 11 b, 12 b, 13 b, 14 b are standard cutsthat are congruent or co-planar with the internal bone interfaceplanes/surfaces of the femoral prosthesis 2 (i.e., surfaces 10 a, 11 a,12 a, 13 a, 14 a). The femoral prosthesis 2 also contains an outersurface 3 that is intended to articulate with a tibial component (notshown) and an optional patellar component (not shown).

Referring back to FIGS. 1 and 2, the robotic guide 50 has a guidingsurface or guiding member for mounting to or positioning relative to abone for use in e.g., a total knee arthroplasty is shown. The roboticguide 50 is disclosed for use in a total knee arthroplasty for exemplarypurposes only and can alternatively be configured as a robotic guidepositioning device for any other joint replacement procedure.

The robotic guide 50 is a bone-mountable robotic guide 50 that includesa motor unit 51 having two axes of rotation 51 a and 51 b. The firstaxis 51 b is coupled to a bone 22 via a bone fixation base 52, and thesecond axis 51 a is coupled to a saw-guide 54. The saw-guide 54 includesa slot (also referred to as a guiding surface) 54 a for guiding acutting tool, such as an oscillating saw-blade (not shown). The roboticguide 50 is capable of positioning the saw-guide 54 at any angle andresection depth such that it can be aligned with any of the femoral bonecuts 10 b, 11 b, 12 b, 13 b, 14 b.

In other words, the robotic guide 50 is e.g., a cutting guide forguiding a cutting tool capable of cutting bone portions at the level ofa head of a femoral bone that includes a seat 52 configured to fasten toa side of the femoral bone adjacent to distal condyles where the femoralbone is to be cut. The seat 52 is coupled to the femoral bone about afirst rotation axis. The robotic guide 50 also includes a means foradjusting in two substantially perpendicular directions the firstrotation axis with respect to the seat including after the robotic guide50 is fastened to the femoral bone. The robotic guide 50 also includesfirst and second motors 51 and an arm supporting the first and secondmotors 51 that are spaced a fixed distance from one another in alloperating positions of the robotic guide, with one end of the arm beingpivotally assembled On the seat 52 according to the first rotation axis.The robotic guide 50 further includes a cutting guide surface 54 aconfigured to guide the cutting tool and is pivotally assembled on thearm according to a second rotation axis where the second motor isoperatively coupled to the cutting guide surface 54 a for rotating thecutting guide surface 54 a about the second rotation axis, where adistance between the first and second rotational axes is also fixed inall operating positions of the device.

The robotic guide 50 is described in more detail in U.S. PatentApplication Publication Nos. 2011/0130761 entitled “Robotic guideassembly for use in computer-aided surgery,” and 2010/0130986 entitled“Surgical robotic system,” and U.S. Pat. No. 8,096,997 entitled “GuidingDevice for Bone Cutting,” the entire disclosures of which are herebyincorporated by reference herein in their entirety.

The robotic guide 50 is a component of the computer assisted surgicalsystem or CAS 10, which can include several additional components. Suchadditional components can include a 3D localizer for measuring or“tracking” the position of tools in 3D space, and software that has thecapability to register patient data, plan the position and size ofimplant, control guide positioning through motion and robot controllers,and check cut surfaces, and so forth. Exemplary computer assistedsurgical systems applicable to the present invention are described inmore detail in U.S. Patent Application Publication Nos. 2007/0106128entitled “Computer assisted surgery system” and 2007/0185498 entitled“System for determining the position of a knee prosthesis,” the entiredisclosures of which are hereby incorporated by reference herein intheir entirety.

In accordance with the present example, the CAS 10 is configured togenerate a bone model of a first bone having a bone interface surface,e.g., a femoral bone 22, without the use of preoperative bone images.Such a bone model can be generated by a computer 100 of the CAS 10configured to deform a generic model of a bone in response to aplurality of positions of a tracking device that are specific to apatient. A detailed discussion of such means to generate a deformed bonemodel is disclosed in U.S. Pat. No. 8,126,533, the entire disclosure ofwhich is hereby incorporated by reference herein in its entirety.

The computer 100 of the CAS 10 is further configured to determine aposition of the implant model of the femoral prosthesis 2 on the bonemodel. Preferably, the computer 100 determines an optimal position ofthe implant model on the bone model. Further details of such means todetermine an optimal position of the implant model on the bone model isdisclosed in U.S. Pat. No. 8,126,533, the entire disclosure of which ishereby incorporated by reference herein in its entirety. The computer100 is further configured to display planned resection contourscorresponding to one of a plurality of predetermined cut configurationson the display, as further discussed below.

The GAS 10 is also configured to determine a position of the roboticguide (i.e., cutting guide) 50 on the bone 22 based on the determinedposition of the implant model on the bone model by the computer of theCAS 10. The robotic guide 50 further aligned or positioned with thestandard cutting planes of the prosthesis (i.e., 10 a, 11 a, 12 a, 13 a,14 a). In the present embodiment, the CAS 10 is also configured toposition the robotic guide 50 or guiding surface 54 a at a cutting planewhich is slightly adjusted/spaced apart from a standard resection. Forexample, the anterior bone resection can be slightly angled (as referredto by reference numeral 15 b in FIG. 4) from the standard anterior boneresection 13 b. Similarly, the posterior bone resection can be slightlyangled (see reference numeral 16 b in FIG. 4) from the standardposterior resection 14 b. The angles of the adjusted resections arepreferably predetermined so that there is a predetermined distance oroffset e.g., at the proximal portion of the end of the cut, i.e., ‘X’ mmfor the anterior cut and/or ‘Y’ mm for the posterior cut, as indicatedin FIG. 4. ‘X’ and ‘Y’ are preferably predetermined to correspond to apreferred amount of press-fit interference, for example in the range of0.05-2 mm, such as about 0.25, 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, and 2.0mm. This results in additional bone remaining under the femoral implantbone interface surface when the femoral implant is inserted intoposition, achieving a desired press-fit interference and stressing thebone that is in contact with the implant to a desired degree.

In sum, the computer 100 has stored in memory a plurality ofpredetermined cut configurations. The plurality of predetermined cutconfigurations is specific to a particular type and size of implant. Thecomputer 100 is configured to position the guiding surface 54 a of therobotic guide 50 or the robotic guide 50 to one of a plurality ofpredetermined cut configurations relative to the bone interface surfaceof the femoral prosthesis 2. Typically the bone interface surfaces areplanar surface, but can be non-planar surfaces as well. Thepredetermined cut configurations are preferably a plurality ofpredetermined planes parallel to or at an angle relative to the boneinterface surface of a prosthesis. In other words, the predetermined cutconfigurations are relative to a bone interface surface of a prosthesisthat remains fixed in position at its optimum position on the bone.Thus, the overall position of the prosthesis from its optimum positiondoes not move when repositioning the robotic guide 50 from onepredetermined cut configuration to another. The plurality ofpredetermined planes parallel to the bone interface plane are preferablyco-planar to the bone interface plane to provide a congruency fit oroffset from the bone interface to provide an interference fit.

The computer 100 i.e., a system controller or processor, has a firstmodule configured to allow a surgeon to select from a plurality ormultiple cut configurations for any one particular size of implant. Theplurality of or different cut configurations corresponds todifferent/various degrees or levels of press-fit interference of aprosthesis on bone. An example of a set of cut configurations isillustrated in FIG. 6. Here, each of the plurality of predetermined cutconfigurations correspond to a standard distal cut, anterior chamfercut, posterior chamfer cut, and an adjusted anterior cut 15, 17, 19 andan adjusted posterior cut 16, 18, 20 of a distal femur for receiving afemoral prosthesis 2. For example, one cut configuration can includeadjusted or offset cuts 15 and 16, and standard cuts 10 b, 11 b and 12b. This cut configuration could be identified as “Press-fit OptionB—0.25 mm” (where 0.25 mm denotes the amount of offset at the proximalend of the cut from the standard cut, in millimeters). Anotherconfiguration having a greater degree of press-fit interference couldinclude adjusted cuts 17 and 18, and this can be identified as“Press-fit Option C—0.5 mm.” Yet another cut configuration having aneven greater degree of press-fit interference could include adjustedcuts 19 and 20, “Press-fit Option D—0.5 mm.” Alternatively, the adjustedresections may be arranged such that they are parallel to the standardcuts but offset by a given distance. For example, adjusted cuttingplanes 15, 17 and 19 can be parallel to the anterior resection 13 b inthe sagittal plane, but offset by the various distances such as 0.25 mm,0.50 mm and 0.75 mm, respectively. The adjusted posterior resectionscould be parallel to and offset from the standard posterior resection ina similar manner. This would provide a more uniform degree of press fitalong the surfaces of the anterior and posterior resections. Other bonecuts could also be adjusted in either their angle or offset (i.e.,parallel but separated by a distance), such as the distal cut oranterior chamfer cut or posterior chamfer cut. Any combination ofstandard and adjusted bone cuts could be envisioned and applicable tothe present invention.

In other words, the CAS 10 is configured to store in memory dimensionaldata of an implant e.g., femoral, tibial and patellar implants for usein a total knee arthroplasty. In addition, the CAS 10 is configured tostore in memory various offset settings that correspond to various“Press-fit Options” to be displayed to the user. The user then selectsone of the various “Press-fit Options” displayed for positioning therobotic guide 50 to desired locations corresponding to a desired degreeof offset or press-fit interference selected/inputted to the CAS 10 bythe user. Alternatively, the user could define a custom “Press-fitOption” by entering the desired angular degree of offset(s) or press-fitinterference(s) that may not be included amongst the predefined“Press-fit Options.” Input to the CAS 10 is preferably accomplished byinput selection via a display 30 in communication with the CAS 10.

In sum, the CAS 10 is configured to intra-operatively receive aselection of one of the plurality of predetermined cut configurationsthat correspond to various degrees of interference fit of a prosthesison bone and position the cutting guide to a position corresponding tothe selected predetermined cut configuration. The CAS 10 can alsointra-operatively receive one or more, e.g., a second, selection of oneof the plurality of predetermined cut configurations and position thecutting guide to a position corresponding to the second selectedpredetermined cut configuration. The user can intra-operatively selectand input to the CAS 10 one of the plurality of predetermined cutconfigurations via an input device, such as touch screen, switch or anyother input device readily known in the art.

Furthermore, the CAS 10 includes a memory having a plurality of implantmodels and a set of default user preferences that corresponds to aninterference fit of a prosthesis of a bone for resecting the bone basedon a selected implant or implant model, as further defined below. Thedisplay 30 also provides a displaying means for displaying the pluralityof predetermined cut configurations. The computer 100 is furtherconfigured to allow a user to intra-operatively select one of theplurality of predetermined cut configurations displayed on the display30.

The CAS 10 can also include a module configured to allow the user (e.g.,a surgeon) to select the desired degree of press-fit interference theyprefer to use from a number of possible configurations (options). Theability to select and/or change the degree of press-fit interference canbe performed intra-operatively i.e., at different stages of the surgicalprocedure (i.e., the different stages of the computer assisted surgicalprotocol). For example, the surgeon may have a personalized profile ordefault user preferences stored into the system, such that when theirprofile is loaded their preferred press-fit option is automaticallyselected. i.e., a set of default user preferences for resecting bonebased on a selected implant or implant model. The module preferablyoffers the ability to change the preselected option at the beginning ofthe surgery, or at other points thereafter, such as during the planningstages of the implant, or after making one or more of the bone cuts.This can be accomplished by inputting a desired “Press-fit Option” tothe CAS 10 via e.g., a display, keyboard, and the like.

The module is also configured to allow a user to go back and change thedegree of press-fit interference after the bone cuts are made, and toreadjust and recut certain areas of the resected bone. For example, thesurgeon might initially choose “Press-fit Option D—0.75 mm.” Afterperforming some or all of the resections with this option, the surgeonmay decide to intra-operatively evaluate a quality of the bone or thefit of the implant on the resected bone, e.g., by using a trialcomponent prosthesis. In doing so, he may find that the fit is too tightand the component is too difficult to impact on the bone. That is, hecan determine if a repositioning of the cutting guide from the defaultuser preference is required. Therefore, he may decide to change thedegree of press-fit interference by selecting Press-fit Option C or B,or the standard cut option A (i.e., no press-fit), and then go back andrecut the femur. In this case the robotic guide 50 would be repositionedor the cutting guide surface repositioned at the cut locationcorresponding to the newly selected press-tit option i.e., one of theplurality of predetermined positions corresponding to varying degrees ofinterference fit between the implant model and bone model. The surgeoncould then recut and remove additional bone, thereby decreasing thetightness/degree of the press-fit interference.

In other words, after a bone resection is made, the user can input intothe CAS 10 another desired degree of bone resection based upon anevaluation of the quality of the resected bone or the assessed press-fitinterference between the implant and resected bone. That is, the surgeonselects another degree of interference fit between the implant model andthe bone model based on the intra-operative evaluation of the resectedbone. Upon receipt of the input from the user, the CAS 10 repositionsthe robotic guide 50 to a subsequent position corresponding to theinputted desired degree of bone resection based upon the assessedpress-fit interference. That is, the computer 100 is configured to allowa user to align the guiding surface 54 a of the robotic guide 50 i.e.,cutting guide, to another one of the plurality of predetermined cutconfigurations after a cut, such as a first cut, of the bone iscompleted.

The CAS 10 also permits the surgeon to directly assess the quality ofthe patient's bone before deciding the degree of press-fit interferenceto apply, or whether to apply any at all, and to use the bone qualityinformation/data in making his selection for the degree of press-fitinterference to apply. The surgeon can assess bone qualityintra-operatively at any stage during the procedure. For example, thesurgeon can evaluate the bone quality after one or more cuts have beenmade. While making the cut, the surgeon can feel the progression of thesaw-bade through the bone as he is pushing on the saw in order to get afeel for how hard or soft the bone is. He may also evaluate the exposedcancellous bone on the cut surface, using visual and tactile senses(such as a figure tip) to gauge the strength and porosity of thecancellous bone. Alternatively, the surgeon may use an instrument toassess the quality of the bone. The tool could be a simple tool such asan awl or drill that is used to indent or puncture the bone or ahardness testing/indentation tool that may provide a quantitativeassessment of the bone properties. Other examples of systems that can beused to assess the quality of cartilage and bone are described in detailin U.S. Patent Application Publication No. 2010/0256504, the entiredisclosure of which is hereby incorporated by reference herein.

In other words, the surgeon assesses the desired degree of boneresection for the particular patient based upon bone quality data orassessment of bone quality of the patient. Based upon the bone qualitydata, the surgeon inputs into the CAS 10 the desired Press-fit Option toapply to the patient, i.e., the user intra-operatively selects anotherdegree of interference fit between the implant model and the bone modelbased on the intra-operative evaluation of the resected bone.

The CAS 10 can also include an intraoperative imaging modality or sensorto sense the bone quality in the operating room. The intraoperativeimaging modality or sensor may provide an objective reading of the bonequality to the surgeon. Such a sensor could be configured to be incommunication with the CAS 10 and emit and receive ultrasonic, infrared,ionizing radiation, or other waveforms known to be used for sensing bonequality or density. Such sensors are known in the art and a detaileddescription of their structure, function and operation is not necessaryfor a complete understanding of the present invention.

Alternatively, preoperative imaging modalities may be used to assess thebone quality or density of bone, such as radiographs (x-rays), dexascans, X-ray computed tomography (CT) scans, and magnetic resonanceimaging (MRI). Any imaging modality may be used or multiple modalitiesmay be combined, including pre-operative imaging and intra-operativesensing.

The surgeon may choose to take into account any one or a variety ofpatient factors when making a choice of which level of press-fitinterference to use and input to the CAS 10 on the particular patientthey are operating on. These may include, but are not limited to: thequality of bone (e.g., hardness, brittleness, density, color, etc.); themorphology of bone and amount of implant-bone coverage (area and shapeof cut-surfaces in contact with bone); type of disease (osteoporosis,arthrosis, rheumatoid arthritis, osteo-arthritis, post-traumatic OA,etc.); age; gender; Body Mass Index (BMI); patient history; activitylevel; susceptibility to pain or degree of pre-operative pain; and bonequality indicators from pre-op imaging: dexa scan, CT grey-level,Hounsfield units (HU), PET, MRI, etc.

In another aspect, the present invention provides a CAS 10 having animplant planning module configured to allow a surgeon to visualize theplacement of an implant component on the resected bone surface e.g., viamodels on a display. Implant placement planning can be performed basedon pre-operative imaged-based models, but is preferably performed viaintra-operative imageless generic models that are registered to thepatient using non-linear morphing techniques. Details regardingimage-free registration and bone morphing techniques are disclosed indetail in U.S. Pat. No. 8,126,533, the entire disclosure of which ishereby incorporated by reference herein.

Referring now to FIG. 7, a portion of a femoral implant planning screen60 of the CAS 10 is shown. The femoral implant planning screen 60 can beshown on a display 30 (FIG. 1) of the CAS 10. The implant planningscreen 60 contains one or more views of a bone model e.g., an anteriorview of a distal femur bone model 61, as well as the planned implantplacement for a given size and position of an implant model e.g., afemoral implant component model 62 on the bone model 61. The planningview also shows the contours of the resected bone model 63 and thecontours or perimeter of the implant model 62. The bone-cut contours canchange based on the patients' native morphology and the planned positionof the femoral implant model 62. Thus, by evaluating the bone-cut andimplant contours the surgeon can assess the amount of bone coveragebetween the implant and bone-cut surface and incorporate thisinformation in choosing how much press-fit interference to apply betweenthe implant and patient's resected bone.

In other words, the surgeon/user can assess the data displayed by theCAS 10 regarding the planned placement of an implant model to a bonemodel. Then, based upon the surgeon's assessment of the plannedplacement data, the surgeon can then input to the CAS 10 a desiredpress-fit interference setting for press-fitting the implant to apatient's resected bone.

The CAS 10 is also configured to display on the display 30 arepresentation of a degree of interference between the implant model andbone model. The representation is preferably a color map where variouscolors of the color map are representative of varying degrees ofinterference. Referring to FIG. 7, the color map 65 on the implantplanning screen 60 is indicative of a degree/level or intensity (i.e.,deformation or strain) of press-fit interference between the bone modeland the implant model based on the selected level of press-fitinterference. The color map 65 can be based on the amount of offsetbetween the standard cut and the press-fit cut for a particular implant,and it can be superimposed directly over the modeled bone cut-surfaces63 and updated in real-time when a new level of press-fit interferenceis selected (for example, by pressing a button 69 on the touch screen tocycle through the different press fit configurations). A legend or key66 may also be included to indicate to the surgeon which colorscorrespond to the various levels of press-fit interference or predictedstrain (i.e., grey=0 mm, pink=0.25 mm, red=0.5 mm, blue=0.75 mm, and soon). The surgeon can then use the contact surface area between the honeand implant to decide how much press-fit interference should be applied(for example if the surface of the anterior cut is small and narrow dueto the natural morphology of the distal femur, the surgeon may choose toapply more press fit interference to compensate for the smaller amountof coverage). Conversely, if there is a relatively large amount of bonecoverage he may choose a smaller amount of press-fit interference inorder to prevent the implant from fitting too tightly or to preventproblems due to insertion of the implant.

The CAS 10 can also include a stress modeling or finite element analysismodeling (FE or FEA) module configured to model and evaluate a degree ofthe press-fit interference of a given implant model and bone model. TheFEA model can be patient specific (i.e., derived from pre operativeimaging, such as CT or dexa, or intra-operative imaging) or generic(morphed, from a single generic FE model or database of models) andinclude measured or inferred values for the mechanical properties of thebone. Bone quality indicators obtained from the intra-operative sensorcan also be incorporated in the model. The stress distribution at thebone implant interface can then be modeled as a function of themechanical bone properties and the level of press-fit interferenceselected. The stress distribution can also be displayed directly in realtime on the planning screen 60 in the form of a color map 65, asdiscussed above and shown in FIG. 7, with the legend values given in MPaor other appropriate units. Thus, the stress model and implant planningscreen 60 can be used to help the surgeon visualize the tightness ordegree of the fit and to select the optimal degree of press-fitinterference for a particular patient, i.e., be patient specific.

The CAS 10 is also configured to navigate an implant impactor (notshown) i.e., a tool used to impact an implant onto a resected bone), andto determine a final position of the implant relative to a planned ormeasured bone cuts. The implant impactor can be integrated into andcontrolled by the CAS 10 e.g., similar to 3D positioning elements of theCAS 10 navigation system.

The CAS 10 can also be configured for use with custom designedprostheses which are custom designed and manufactured for eachindividual patient (such as those marketed by ConforMIS Inc. ofBurlington, Mass.). Such custom implants can be designed based onpre-operative images and generated models of the patient's bones. Theamount of press-fit interference can also be custom designed into theprosthesis based on all the above mentioned parameters, including anassessment of the patient's bone quality.

Referring now to FIG. 8, a flowchart representing an example of a methodfor preparing a bone for an implant during an arthroplasty in accordancewith the present invention is shown. The method includes inputting andloading surgeon preferences regarding desired degrees of press-fitinterference for a given type of implant (e.g., TKA implant) into theCAS 10 (step 100) along with data on various implant models having abone interface. A bone model is then generated by the CAS 10 or thepatient's bone anatomy is registered, to which an implant is to beimplanted (step 102). The CAS 10 then plans or determines an optimumsize and location of the implant to be implanted using the generatedbone model and implant model along with user preferences as a suggestedinitial plan for the user (step 104). That is, an optimum position ofthe implant model on the bone model is determined. Thereafter, thesurgeon intra-operatively evaluates and assesses a quality of the boneof the patient to which the implant is to be implanted. Afterwards, thesurgeon selects a degree of press-fit/interference fit to be used for aselected implant and validates the plan for size and location asinitially determined by the CAS 10 (step 106). In other words, a degreeof interference fit between the implant model and the bone model basedon the intra-operative evaluation of the quality of the bone isselected. If no further or additional adjustments to the plan arerequired, the surgeon mounts/positions a robotic guide 50 having acutting guide surface on the patient's bone to a position correspondingto the planned and selected degree of press-fit/interference (step 108)i.e. based on the determined optimum position.

After the robotic guide 50 has been mounted to the patient's bone thecutting guide surface is positioned to one of the plurality ofpredetermined cut configurations that corresponds to the selected degreeof interference fit. The surgeon then resects one or more portions ofthe bone in order to conform the bone to receive the selected implantfor implantation thereto (e.g., a distal femoral resection.) Then thesurgeon evaluates and assesses the quality of the bone, and inparticular a quality of the resected bone. Thereafter, the surgeoninputs into the CAS 10 a updated selection for a degree of press-fitinterference to be used based on the bone quality evaluation after aninitial resection has been performed (step 110) and repositions therobotic guide 50 to a position corresponding to the updated degree ofpress-fit selected (step 112). In this step, the entire robotic guide 50can be repositioned or a cutting guide surface 54 of the robotic guide50 repositioned. Thereafter, the user resects a subsequent portion ofthe bone (e.g., an anterior, a posterior, and/or a chamfer resection inthe case of a TKA) (step 114).

After all the bone cuts have been made, the surgeon can evaluate the fitof the implant on the resected bone and tightness or degree of fitduring impaction of the implant or trial implant to the resected bone(step 116). Based on this assessment, the surgeon can adjust the degreeof press-fit interference and update the plan if the fit of the trialcomponent is determined to be too tight (step 120). If any adjustmentsis necessary, the surgeon then proceeds to adjust the cutting guideaccording to an updated plan (step 112) and recuts the bone to achieve abetter fit (step 114). After all adjustments have been made, and thesurgeon has determined that no further adjustments are necessary, thesurgeon proceeds to impact the implant on the bone and completes thesurgical procedure as usual (step 122).

In addition to the above described embodiments and features of thepresent invention, the CAS 10 can be configured in a number ofvariations. For example, autonomous or haptic robotic guides a can beused to machine or burr out the bone, such as those systems developed byMAKO Surgical Corp. or the ROBODOC system, previously marketed byIntegrated Surgical Systems (ISS) of Fremont, Calif. Further, bonepreparation can be performed by any known method, including sawing,milling, or laser/water jet cutting. The CAS 10 can employ any type oftracking (localization) system, including magnetic, infra-red,ultrasonic, or gyro-accelerometer based systems. The CAS 10 can also beapplied to resurfacing implants with curved or curvilinear shaped cuts,such as those used in uni-compartmental arthroplasty or total kneeresurfacing.

The systems and methods of the present invention can also be applied toany type of implant (partial and total resurfacing, custom implants) andto other joint replacement procedures (knee, hip, ankle, shoulder,elbow, etc.)

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

I/We claim:
 1. A computer assisted surgical system for guiding bonecutting operations of an arthroplasty surgery comprising: a cuttingguide to be positioned relative to a first bone; and a computer having aplurality of predetermined cut configurations corresponding to variousdegrees of interference fit of an implant on the first bone, and whereinthe computer is in communication with the cutting guide and configuredto position the cutting guide to one of the plurality of predeterminedcut configurations.
 2. The system of claim 1, wherein the plurality ofpredetermined cut configurations is relative to a bone interface surfaceof the implant.
 3. The system of claim 1, wherein the computer isfurther configured to generate a bone model of the first bone withoutthe use of pre-operative bone images, determine an optimum position ofan implant model having a bone interface surface on the bone model, anddetermine a position of the cutting guide relative to the first bonebased on the determined position of the implant model on the bone model.4. The system of claim 1, wherein the computer is further configured tointra-operatively receive a selection of one of the plurality ofpredetermined cut configurations and position the cutting guide to aposition corresponding to the selected predetermined cut configuration.5. The system of claim 1, wherein the plurality of predetermined cutconfigurations is a plurality of predetermined planes parallel to or atan angle relative to a bone interface surface of the implant.
 6. Thesystem of claim 5, wherein the plurality of predetermined planesparallel to the bone interface surface is co-planar to the boneinterface surface to provide a congruency fit or the plurality ofpredetermined planes is offset from the bone interface surface toprovide an interference fit.
 7. The system of claim 1, furthercomprising a display for displaying the plurality of predetermined cutconfigurations.
 8. The system of claim 7, wherein the computer isfurther configured to display planned resection contours correspondingto one of the plurality of predetermined out configurations on thedisplay.
 9. The system of claim 7, wherein the display includes arepresentation of a degree of interference between the implant model andbone model.
 10. The system of claim 9, wherein the representation is acolor map and various colors of the color map are representative ofvarying degrees of interference fit.
 11. The system of claim 7, whereinthe computer is further configured to allow a user to intra-operativelyselect one of the plurality of predetermined cut configurationsdisplayed on the display.
 12. The system of claim 1, wherein thecomputer is further configured to allow a user to align the cuttingguide to another one of the plurality of predetermined cutconfigurations after an initial cut of the first bone.
 13. A method ofpreparing a bone for an implant during an arthroplasty comprising thesteps of: providing a computer assisted surgery system having aplurality of predetermined cut configurations; selecting a degree ofinterference fit between an implant and a bone that corresponds to oneof the plurality of predetermined cut configurations; and positioning acutting guide to one of a plurality of predetermined cut configurationscorresponding to the selected degree of interference.
 14. The method ofclaim 13, further comprising the steps of: generating a bone model of abone; providing an implant model having a bone interface; anddetermining an optimum position of the implant model on the bone model,wherein the cutting guide is positioned relative to the bone based onthe determined optimum position.
 15. The method of claim 13, whereinselecting the degree of interference fit between the implant and bone isbased on an intra-operative evaluation of a quality of the bone.
 16. Themethod of claim 13, further comprising the steps of: resecting the boneat a plane corresponding to the positioned cutting guide:intra-operatively evaluating a quality of the resected bone; andselecting another degree of interference fit between the implant modeland the bone model based on the intra-operative evaluation of theresected bone.
 17. The method of claim 14, wherein the bone model isgenerated without the use of pre-operative bone images.
 18. The methodof claim 14, wherein the bone model is generated using a computerassisted surgical system in communication with and operably connected tothe cutting guide.
 19. A method of preparing a first bone for an implantduring an arthroplasty comprising the steps of: providing a computerassisted surgery system having a set of default user preferences forresecting bone that corresponds to an interference fit of a prosthesison the first bone; positioning a cutting guide in communication with thecomputer assisted surgery system relative to the first bone; aligningthe cutting guide to correspond to the default user preferences forresecting bone; intra-operatively evaluating a quality of the firstbone; and determining if a repositioning of the cutting guide from thedefault user preferences is required.
 20. The method of claim 19,wherein the computer assisted surgery system has a plurality of implantmodels in memory and is further configured to generate a bone model ofthe first bone, and wherein the method further comprises the step ofselecting an implant model and determining an optimum position of theimplant model on the bone model.